Preparation Of Transition Metal Single Atomic Site Catalysts For Electrochemical Oxygen Reduction Reaction

Single atomic site catalysts（SACs）have recently attracted increasing research attention because of their unique catalytic properties.With improvements in synthetic methods,numerous SACs have been fabricated successfully.SACs exhibit excellent performance in electrocatalysis and are also highly active in other catalytic reactions,thus,they are considered potential successors to homogeneous and heterogeneous analogues.For one thing,it is widely known that the catalytic performances of heterogeneous catalysts is highly dependent on support materials.Optimal support materials can shorten diffusion pathways and provide sufficient interfacial area that is accessible to reactants.General synthetic methods are required to develop SACs on various support materials,and thereby broaden the applications of these promising catalysts.On the other hand,oxygen reduction reaction（ORR）,as an important cathodic process in energy conversion systems,has received considerable attention in recent years.The sluggish kinetics of ORR is one of the most important issues in energy-related devices and thus increase demand for the development of high-performance electrocatalyst.Pt or Pt-based materials are the commonly used electrocatalysts for ORR,but their applications are severely affected by the high cost and limited availability.Moreover,the poor long-time stability and methanol crossover effect of Pt can further restrict the widespread applicability in ORR.Therefore,it is meaningful to design highly efficient alternative catalysts,involving non-noble metals instead of Pt-based catalysts.In this paper,transition metal single atomic site catalysts with excellent performance of ORR were prepared by simple pyrolysis strategy and the relationship between catalytic performance and site was studied.Main conclusions are listed as following:In this study,we have successfully prepared a high-performance isolated single-atomic copper site-based catalyst（Cu₁NC）with different degrees of carbon defects via direct pyrolysis of preorganized hexaazatriphenylene precursor（HAT-CN₆）at different temperatures.Experimental results suggested that the as-prepared Cu₁/NC-900 catalyst shows excellent ORR catalytic activity with a half-wave potential(E_1/2)of 0.894 V,kinetic current density（J_k）of 35.12 m A cm^-2at 0.85 V（vs.RHE）,and Tafel slope of 45 m V dec^-1in alkaline media.Furthermore,the Cu₁/NC-based Zn-air battery with 6.0 M KOH electrolyte exhibits higher power density,current density,rechargeability and cyclic stability than the one based on Pt/C,which demonstrates that Cu₁/NC-900 is an excellent non-noble metal catalyst for boosting ORR towards Zn-air batteries.Theoretical results showed that Cu-N₃sites decorated by the neighboring carbon defect are more contributed to immediate desorption.Single-atomic Fe sites catalyst supported on nitrogen-doped porous carbon was successfully synthesized by double template method using silica nanopartcles and mesoporous g-C₃N₄as templates and zinc carbonate as carbon source.The prepared single-atomic Fe sites catalyst exhibited a higher half wave potential of 0.904 V in alkaline media for ORR,far exceeding the performance of commercial Pt/C,and showed methanol tolerance and cycling stability.In addition,the single-atomic Fe sites catalyst showed excellent cycle stability of charge-discharge,and low charge-discharge voltage gap for zinc-air batteries.